1. Statement of the Technical Field
The invention concerns dielectric substrates for RF circuits, and more particularly dielectric substrates with selectively tailored electrical properties.
2. Description of the Related Art
RF circuits, including antennas, are commonly implemented on dielectric substrates. Materials commonly used for this purpose include commercially available low and high temperature cofired ceramics (LTCC, HTCC). For example, low temperature 951 cofire Green Tape™ from Dupont® is Au and Ag compatible, has a acceptable mechanical properties with regard to thermal coefficient of expansion (TCE), and relative strength. It is available in thicknesses ranging from 114 μm to 254 μm and is designed for use as an insulating layer in hybrid circuits, multichip modules, single chip packages, and ceramic printed wire boards, including RF circuit boards. Similar products are available from other manufacturers.
LTCC substrate systems commonly combine many thin layers of ceramic and conductors. The individual layers are typically formed from a ceramic/glass frit that can be held together with a binder and formed into a sheet. The sheet is usually delivered in a roll in an unfired or “green” state. Hence, the common reference to such material as “green tape”. Conductors can be screened onto the layers of tape to form RF circuit elements antenna elements and transmission lines. Two or more layers of the same type of tape is then fired in an oven. The firing process shrinks all of the dimensions of the raw part. Accordingly, it is highly important that the material layers all shrink in a precise, well defined way that will provide consistent results from one module to the next.
Other materials commonly used as RF substrates include Teflon® PTFE (PolyTetraFluoroEthylene) composites of glass fiber, woven glass and ceramics. Such products are commercially available from a variety of manufacturers. For example, Rogers Corporation of Chandler, Ariz. offers such products under the trade name RT/duroid including product numbers 5880, 6002, and 6010LM. Unlike LTCC materials, these types of substrates do not generally require a firing step before they can be used. Instead, they are typically provided in the form of rigid board material with a conductive metal ground plane formed on one side.
Due to the maturing of the antenna design process, the continued improvement of new antennas is most limited by the choice of substrate materials. However, development of new materials has proved difficult for a variety of reasons. One reason concerns certain incompatibilities of the physical properties associated with diverse materials that would otherwise be desirable to combine in a single dielectric composition. Often, the thermal coefficient of expansion (TCE), chemical properties of the materials, or sintering properties of the material may be inconsistent with one another. For example, different types of unfired ceramics such as Green Tape™ will not fire well together because of different chemical and physical properties of the various different types of materials.
Still, new materials are needed for a variety of reasons. One such reason relates to the limited variety of specific electrical properties that are offered in commercially available dielectric substrate materials. Designers wishing to implement antennas or other RF circuit designs often find themselves constrained by the limitations of the material. For example, it may be desirable in a particular instance to implement an antenna array on a portion of a substrate having a particular value of permittivity, permeability or loss tangent. The requirements for these electrical properties can relate to form factor, electrical performance or other design issues. In any case, the limited choices of substrate materials that are presently available can require design compromises that are preferably avoided.